This invention relates to spacer for the glass panes in a sealed window unit, to a method for manufacturing the spacer and to a sealed window unit including the spacer.
Sealed window units of this type have been known for many years and have become increasingly important and popular as energy conservation and energy efficiency have become more popular. Such a sealed window unit comprises an outer frame or tape supporting the glass panes and a spacer between the glass panes. The spacer is formed in strips which lie along adjacent edges of the panes so as to define a space therebetween and to support the panes in the defined spacing.
To date the most common material which has been commerically used for the spacer in the manufacture of such units has been steel. Steel has been used mainly exclusively because it has a co-efficient of expansion similar to that of glass and because this property is the most important in the manufacture of such a unit. It will of course be appreciated that any difference in expansion particularly in climates which have large changes in temperature can have many disastrous consequences, particularly as size of the window increases, including cracking of the glass and at least breaking of the seal between the panes of glass. Other metals such as aluminum are completely unsatisfactory on medium to large windows in that the thermal expansion is completely different from that of glass. Similarly many plastics materials such as nylon, vinyl, polythene are available but again these are completely unsatisfactory in view of very different expansion characteristics. To date therefore steel has been the generally accepted material even though this has a number of considerable disadvantages. In particular, the thermal conductivity of steel is considerably higher than that of glass or of the air space between two panes of glass. In a sealed unit heat from within a building tries to escape from a building and the path it takes is through the path of least resistance. In the case of a sealed window unit, the path of least resistance is around the perimeter of the unit where the steel spacer strip is provided. Thus heat is rapidly lost from around the perimeter of the window often causing a ten degrees to twenty degrees farenheit temperature drop at the perimeter of the window relative to the center thereof.
This temperature differential results in differential shrinkage between the center of the glass pane and the perimeter. This can result in a stress crack developing in the glass or with result in the loss of the sealing around the edges of the panes. When the seal breaks down outside air can enter the space between the windows carrying water vapour which is deposited inside the panes causing fogging of the window unit. Approximately five percent of the window units manufactured tend to fail due to such stress cracks, or loss of seal. However it is often thought that the failure is due to shifting of the building rather than to a failure of the window unit itself.
A yet further problem with steel spacer strips is that they are manufactured in certain cut lengths which necessarily cause wastage when cut to specific lengths for use in the window unit. Obviously it is necessary to cut the steel into particular lengths for shipping and handling and of course these lengths cannot be predetermined relative to particular requirements. In addition the finite lengths of the steel strips makes automation of the handling and cutting processes more difficult.
Steel and aluminum spacers of this kind are manufactured by rolling or folding to form a hollow body which is substantially rectangular in cross-section with a slot or interlock along the upper surface between the two folded edges of the steel strip from which the folded body is formed. It is important to maintain the width of the slot as narrow as possible in order to allow the ingress of air from the space between the window panes while preventing the escape of granular dessicant material. In manufacture of a window unit using the spacer, dessicant is added into the interior of the hollow spacer so as to dry out the air remaining between the panes of glass so that no water is condensed onto the inside faces of the glass thus fogging the window unit. Steel which is very unsuitable for extrusion processes can however be readily formed by a folding process into a shape of this type. However folding processes are limited in the type of shape that can be formed.